Leader of the Islamic Revolution Ayatollah Seyyed Ali Khamenei addresses the members of Iran’s Assembly of Experts, Tehran, March 12, 2015.

TEHRAN —Iran’s highest leader issued a sharp response Thursday to a letter to the country’s leadership by Republican lawmakers, deriding it as an indication that Washington is “disintegrating” from within.

AyatollahAli Khamenei,Iran’s supreme leader, said the letter warning that any nuclear deal could be scrapped by a new president was “a sign of a decline in political ethics and the destruction of the American establishment from within.” The statement was posted on his website.

Ayatollah Khamenei, who will have the final say in Iran over a nuclear deal, characterized the open letter written by 47 Republican senators on Monday as a reflection of Washington’s decadence.

“All countries, according to the international norms, remain faithful to their commitments even after their governments change, but the American senators are officially announcing that at the end of the term of their current government, their commitments will be considered null and void,” Ayatollah Khamenei wrote.

Most surprising perhaps was the fact that Ayatollah Khamenei appeared to continue to support the nuclear talks, despite the Republicans’ threats that they, or a possible Republican president in 2017, would try to undo any deal made.

Ayatollah Khamenei has often expressed doubts about American intentions and sincerity in the negotiations, but on Thursday he supported Iran’s negotiating team, as he has frequently in recent weeks. Speaking to members of the Assembly of Experts, a council that theoretically has the constitutional authority to appoint and dismiss the supreme leader, he called them “good and caring people, who work for the country.”

In Iran, as in the United States, conservatives tend to distrust the other side in the negotiations and are thought to prefer that the whole effort come to nothing. But as long as the supreme leader is voicing support for the talks, criticism from the hard-liners will be muted.

Nevertheless, Ayatollah Khamenei warned that it remains important for Iran’s team to be vigilant, saying that when deadlines in the talks loom, the opponents harden their positions. “Of course, I am worried because the other side is a sneaky and crafty one who stabs with a dagger in your back,” he said.

Ayatollah Khamenei also noted that the recent speech to Congress by the Israeli prime minister, Benjamin Netanyahu — whom he referred to as a “Zionist clown” — had led to division in Washington. “Some American officials, who did not want to side with him, made some remarks,” he wrote.

He repeated remarks that the Islamic State militant group, referring to it by its Arabic name, Daesh, is an American invention.

“America and its allies in the region have created the most evil and wicked terrorists like Daesh, but accuse Iran of committing such acts,” he said.